This invention relates to a method and apparatus for distinguishing natural diamond from synthetic diamond by observing the luminescence, for example by investigating the arrangement of growth sectors in the diamond.
Synthetic diamonds differ from natural diamonds in that synthetic diamonds show mixed habit growth, having different growth sectors through the body of the crystal. These different growth sectors all incorporate impurities as they grow, but do so at different rates and in different ways, and this gives them different spectroscopic properties. In an unpolished stone, the presence of different growth sectors in synthetic diamonds will produce characteristic growth faces on the surface of the diamond quite distinct from those formed on the surface of a natural diamond, which will tend to exhibit single habit octahedral growth. A skilled person can identify an unpolished synthetic diamond from a natural diamond just by looking at it, but all this surface information is removed if the stone is polished.
A technique known as cathodoluminescence has been developed and discussed in Woods & Lang (J. Crystal Growth vol 28 (1975) page 215), Burns et al (J. Crystal Growth vol 104 (1990) page 257), Shigley et al (Gems and Gemology, vol 23 (1987) page 187), and Marshall ("Cathodoluminescence of Geological Materials", 1988, published by Unwin Hyman, pages 19 to 36). This technique involves subjecting a diamond to an electron beam in an evacuated cathode ray chamber. With a typical energy of 18 keV, electrons will penetrate and cause excitation in a surface region to a depth of about 3 .mu.m. Luminescence known as cathodoluminescence will be generated in this region. An image of the surface being excited can be then formed, this image showing the cathodoluminescence pattern.
Ponahlo (J. Gemology, vol 21 (1988) page 182) describes the use of the cathodoluminescence technique for distinguishing natural from synthetic emeralds and rubies.
The cathodoluminescence technique has significant disadvantages in the form of the apparatus required. The gemstone has to be placed in a vacuum chamber, which is expensive and increases the time required to make a measurement, and the electron beam generates X-rays which have to be screened. In addition, the cathode ray apparatus itself is expensive.
It is desirable to provide a method of distinguishing natural diamond from synthetic diamond without using complex and expensive apparatus or vacuum chambers.
Shigley et al discloses a method of short wave ultraviolet illumination of synthetic diamonds to study growth sectors, using ultraviolet radiation of a wavelength of 254 nm. This causes excitation into impurity energy level(s) specific to the particular type of diamond being studied. The technique described in this disclosure will only work with those diamonds that have a strong extrinsic absorption at 254 nm.
It is desirable to provide an observation technique that will work with a single wavelength or single band of wavelengths for all diamonds.
A paper by Walsh et al (Journal Of Luminescence, Volume 4 (1971) page 369) describes the thermoluminescence technique in natural and synthetic semi-conducting diamonds, and relates it to phosphorescence phenomena. Thermoluminescence is the thermal generation of luminescence following excitation at a low temperature, for example 77K. The sample is excited using an electron beam or photo-excitation from an arc lamp.
Upon raising the temperature of the sample, thermoluminescence peaks at different temperatures are observed. Phosphorescence may also be observed.
Phosphorescence is the decay of stimulated luminescence, following the removal of the excitation source, over a period of milliseconds to tens of seconds and sometimes over a period of minutes.
Thermoluminescence apparatus is complicated and expensive and may not be suitable for differentiating synthetic diamonds and natural diamonds.